Broadcasting with reduced band width



March 1, 1938. N. KOOMANS BROADCASTING WITH REDUCED BAND WIDTH Filed Dec. 14, 1935 PIC-3.2.

FILTER 45o 167 kc FILTER 17 22 kc FILTER CARRIER WAVE Patented Mar. 1, 1 938 V BROADCAS TIhTG;

The mv'enfian" relates toli'as inethodzemote casting: and tozatransmitter! in. which: a; modulatedi wave is: radiated; said wave comprising i as cai rier and two sidebands; onerof saidside bands "being i I completei 'andrthe other: containing only;

components-corresponding to low notes;-

According to the invention such alwave' is: ob-

tained byl 'dividing the voice-current .low 'fre'-.-

quency'= nto two:- parts; one of which: comprises 'only 'high; notes,- the other partcontaining the remaining} low notes. I The: low; notes; are. then: modulated according to the usual: two sid'e band niethodgjan'd the high notes according toithe single si'cle-band method, the--- carrier frequency products of modulation are added'. a

T-heinventionl'is' the outcome of attempts to reduce -the actual over-crowdinginthe" ether.

A' well known 'proposal to obtain such a reduc tion is 'the suppression of one of the. two side bands in; awave modulated} according to the a usual t'vv'oside-bandmethod-E The following-ditficul'tiesz are mainlyassociated'with this=propos '51- 1 First it is difficult to include the low notes to the correct extent in the' mod1'1lation'and -sec ondly distortion products in the 'shape of higher' harmonics occur'in the case oflinear rectification atthe"'receivi-' ng end; thisdefect" increasing as the depth of modulationis'increased: Low notes- -are especially troublesornein this respect in 'thefirstplacebcause'they'are as a rule deeply modulated on the carrier, and secondly because the higher harmonics of these components fall mostly within the audible range.

5 As a rule the higher notes are much less strongly modulated and for the most part these higher notes determine the timbre.

The present invention meets these difiiculties by utilizing a system ensuring a considerable 40 reduction in the necessary band-width, but with which at the same time a good quality of communication is possible. A feature of the method of the invention used in this system is that in addition to the carrier wave one side band is 45 sent out completely and the other side band only insofar as the components represent low notes.

In other words the low notes of the audiofrequency spectrum to be radiated will be represented in both side bands, and the high notes in 50 one side band only.

In this way the distortionless reception of low notes in a linearly detecting receiver is ensured. As the low-note energy is distributed over the two side bands and the high-note energy is con- 55 centrated in a single side band, the ratio of the WITH. REDUGEDa' BAND'L vvm'rn; t

lflicolaas Kooma ns fl lhe Hague. Netherlands,

Application -Decembeis 14, 1935;Seria12No: 54,359. /2'

, Ire the; Netherlands December 14, 1934 5. claim 1101." 250-175 being the same in both cases. Subsequentlyfthe the audio frequencies in: two ranges-,. theone 15:

range only-containing low notes and the other containing the remaining high notes. The first rangeis modulated in the. usual two-side bandmanneraand the-second range according to a single-side band system. The low notes may thus be modulated in ausual manner, which does not 'giverise to difiiculties, and the high notes may be directly modulated on a carrier of comparatively high f requency without consequent' difiicultie's relating to the-filtering off one side bandffromthe resulting mixture; 'I-he said high frequency carrier may be intermediate or alternatively may itself be the carrier t0-beradiatedj dependent on circumstances; such as the'wave-length on which the. modulation is; ef Q:

fected and on the frequency ofseparationof' the high and low ranges.

By the possibility of choosing at a high value the carrier on which the high notes are modulated, any difiiculty due to the appearance of higher harmonics of the modulating notes in the modulation is avoided.

It is advisable to arrange suitably for the two-side-band modulation of the low notes to be flat, so that the single-side-band modulated high notes may be added without carrier wave; in other words the surplus of carrier in the twoside-band modulated part serves as the carrier in the single-side-band modulation. The depth of modulation will be so chosen that a suificient carrier amplitude is obtained for the entire wave this carrier will have the required phase.

As it may be difiicult to efiect a separation into a high and a low region in an abrupt manner and the line of separation will thus in general be oblique, it is desirable to take this effect into account in the modulating process and to secure a corrective overlap of the two obliquely limited note components. 55

The invention will now be described with particular reference to the accompanying drawing,

In Fig. 1 the abscissae represent frequencies and the ordinates amplitudes. The location of the carrier frequency is indicated. This fre-.

Fig. 2 shows a diagram of a transmitteradapted to realize the wave form shown in Fig. 1.

The transmitterof Fig.2 modulates in two stages, the first *carrier ffrequency being 15 kc. and .the second he; modulation is. by an,

audio-frequency. band extending from 0-7, k. c. in. the mannershown in Fig. 1, so that the resulting wave: comprises frequencies extending fr0m,15+1452 k. c. to 15+145+7 k. c., that is from 158-167 k. ,c., the carrier frquency being k. 0; As showntheaudio-frequencies entering at l are amplified in two paralleled amplifiers land. 3, the amplifier 2 being provided with a band-pass filter from 0-2 k. c. and amplifier 3 having'a band-pass filter from 2-7 k. c. I The output from amplifier 2 is ,modulated in modulator :4, theoutput from amplifier'3 in modulator 5,

each-of these modulators being supplied by carrier from generator 6 at- 15 k. c.; the modulators are provided with output band'filters as shown,

whereby the output from modulator 4 comprises two side-bands and that from modulator 5 only 'Ihrough the volume control ll] the outputs from amplifiers ll and 9 are supplied to the final modulator I I, supplied from the generator 1 with carrier at 145 k. c. The output from modulator.

l [is led througha filter l2 and emitted, the frequency spectra of this emission being shown again below the filter.' i

As shown the audio-frequencies from" I claim:

1. Method of broadcasting comprising the steps of amplifying a section of the audio-band to be modulated, this section comprising lower frequencies, and separately amplifying the remaining part of the audio-band comprising higher frequencies, modulating the first-named section in a two-side band modulator and the second section in asingle-sideeband-modulator, amplifying the modulation products to different ampli- V tudes, and adding the modulation products. quency is modulated in the example with audio- 2. Method of broadcasting comprising modulating a section of the audio-band containing only notes below a given frequency in a first modulator on a given carrier frequency and the remaining section of the audio-band on the same carrier in a second modulator, the first modulator being a two-side band modulator and the second modulator a single side band modulator, differently amplifying the modulation products, and radiating the said products. 3. Method of broadcasting comprising modulating a section of the audio-band containing only notes below a given frequency in a first modulator on a low carrier frequency and the re-. maining section of the audio-band on the same carrier in a second modulator, the first modulator being a two-side band modulator and the sec-, ond modulator a single side band modulator, differently amplifying the modulation products, modulatingthe products of modulation of the first and second modulators on a third modulator operating at a higher carrier frequency, and radiating the products'of modulation of this mod ulator.

4. Broadcast transmitter, comprising a path for low audio frequencies and a path forthe remaining audio-frequencies, the first-mentioned path including a double side-band modulator for modulating the low audio-frequencies on a carrier, the second path including a single side-band modulator for modulating the remaining frequencies on a carrier of the same frequency,

' means for differently amplifying the modulation products and means for radiating the products of modulation together. H

5. A-method of broadcasting as set forth in claim 2, additionally characterized in that the single side-band modulator is of the suppressed carrier type. 1 v I NICOLAAS KOOMANS. 

